1. Field of the Invention
This invention relates to an improved ultrasonic imaging apparatus in which image interference due to ultrasonic signal refraction and bending is corrected. It further relates to an improved ultrasonic imaging array design for use with such a system or a system which compensates for time delay errors in ultrasonic signals arising from ultrasonic velocity inhomogeneities in objects being examined, in accordance with the teaching of the above referenced related applications. The invention further relates to such a system incorporating digital electronics which allow image correction and enhancement on a realtime basis.
2. Description of the Prior Art
The use of ultrasonic sound waves in apparatus for the examination of solid objects is now a well known and comparatively well developed art. In such apparatus an array of one or more ultrasonic transducer elements is used to transmit ultrasonic waves into the object, and reflections of the waves from within the object are used to define geometry and related characteristics of the object's interior. Such ultrasonic imaging apparatus has been found to be particularly useful in medical applications as a non-invasive diagnostic tool. The state of the art in such medical applications has been reviewed, for example, by Havlice and Taenzer, "Medical Ultrasonic Imaging," Proceedings of the IEEE, Volume 67, No. 4, Apr. 19, 1979, pages 620 to 641.
The use of a segmented, annular transducer array for ultrasonic imaging apparatus is disclosed in U.S. Pat. No. 4,270,546, for characterizing a preferred direction of fibrous tissue inside a biological structure, but with no suggestion of utilizing such an array structure to compensate for time delay errors.
Proposals have been made in the prior art to utilize digital electronics in ultrasonic imaging apparatus. For example, Waag, R. C., P. P. K. Lee and R. Gramiak, "Digital Processing to Enhance Features of Ultrasound Images," Proc. Ultrasound Symp., IEEE CH 1120-55U, pages 163-167, J. DeKlerk and B. R. MacAvoy, eds. (1976); and Corl P.D., P. M. Grant and J. S. Kino, "A Digital Synthetic Focus Acoustic Image System for NDE," Proc. 1978 IEEE Ultrasound Symp., pages 263-268 (1978), both disclose the use of such electronics in such systems. However, the systems disclosed there are not suitable for realtime correction of time delay errors resulting from inhomogeneities in tissue or other objects being analyzed, nor for the correction of images distorted due to multipath interference of ultrasonic waves generated by the apparatus.
Recognition of the image distortion problems addressed by this invention is contained in Savatius, A. D. et al, "Ultrasonic Beam Distortions Introduced by Rib Cage in Cardiac Investigations," Proc. 26th Ann. Mtg. Amer. Inst. of Ultrasound in Medicine and 10th Ann. Mtg., Soc. of Diagnostic Medical Sonographers, p. 72 (1981), but with no suggestion of correcting for such image distortion.
Thus, while the art of ultrasonic imaging is a well developed one, there remains a further need for such an apparatus which will correct image distortion due to multipath interference, refraction and obstruction of the ultrasonic waves, and for an improved array design for such systems incorporating time delay error correction and ultrasonic wave image distortion or degradation correction.